This invention relates in general to mass transfer and heat exchange columns and, more particularly, to methods and apparatus to improve vapor distribution in such columns.
In mass transfer and heat exchange columns, liquid and vapor streams are brought into contact with each other, normally in countercurrent flow, for mass or heat transfer, fractionation or other operations. Various types of internals, such as trays and random and structure packing, have been developed to facilitate interaction between the liquid and vapor streams within selected contact regions of the column. In order to increase the efficiency of the mass transfer or heat exchange taking place between the vapor and liquid within these contact regions, it is important that the liquid and vapor be uniformly distributed across the horizontal cross section of the column, particularly at the lower vapor-liquid interface where the vapor enters the packing or other contacting device.
In columns of the types described above, vapor or mixed-phase feed streams are frequently introduced radially or tangentially into the column through a feed nozzle at a location below the contact region. The vapor phase of the feed stream then rises through the contact region and interacts with downwardly flowing liquid. Various devices have been developed in an attempt to interrupt the radial or tangential momentum of the vapor feed and redirect it so that it is able to rise in a more uniformly distributed manner across the cross section of the column as well as to separate the liquid components present in the feed stream from the vapor phase. In U.S. Pat. No. 5,106,544 to Lee et al., internal vanes are positioned within an annular vapor horn and are oriented to redirect the vapor or mixed phase feed stream downwardly through the open bottom of the vapor horn. The downwardly deflected vapor is then said to rise in a more uniform manner into an overlying packing bed. In U.S. Pat. No. 5,605,654 to Hsieh et al., several different embodiments of vapor horns are disclosed, including those in which deflectors or anti-swirl baffles are positioned radially inwardly of the inner annular wall of the vapor horn. The purpose of the anti-swirl baffles is to disrupt the cyclonic or swirling vapor flow along the inward face of the inner annular vapor horn wall.
In many types of towers and columns, including but not limited to virgin crude vacuum columns, virgin crude columns, FCCU main fractionator slurry pumparounds, visbreaker vacuum flashers, heavy oil vacuum towers, heavy oil fractionators, coker main fractionators, visbreaker fractionator, flexicoker main fractionators, and recovered lube oil vacuum towers, the vapor or mixed phase feed stream is fed at high velocity through the feed nozzle into a flash zone located just above a section where the column transitions to a reduced diameter. The feed nozzle typically includes a vapor horn or vane inlet device that facilitates a coarse separation of vapor and liquid and is designed to create a uniform distribution of vapor entering the overlying internals. The internals can include trays, random packing, structured packing, grid packing, open spray chambers or side-to-side shower decks. Some of the vapor feed entering the flash zone travels downwardly into the region of reduced column diameter before reversing direction and moving up through the flash zone and into the overlying internals. The motion imparted to the high velocity vapor stream by the vapor horn or other feed device interacts with the transition to the reduced column diameter to create an intense swirling or cyclonic motion in the column section below the feed nozzle. This swirling motion is only partially abated using the anti-swirl baffles on the inner wall of the vapor horn described above, and can cause liquid on the inner wall of the column and any underlying internals to become entrained in the vapor. The swirling motion also inhibits the ability to uniformly distribute the vapor into the internals overlying the flash zone.
A need has thus developed for a method and apparatus to further reduce or eliminate the swirling vapor motion in the columns described above.